I tried the five-day Roomster trial. I live in New York City, so I figured the site would have some legit listings as it can be a tough city to find housing. For every one real listing there are 15 scam listings. I'd also like to note that there are not many real users / listings on here.
I wish Roomster would take an active role is cleaning up the false listings. You can tell they are fake because 1. the rent prices are insanely low 2. the user immediately asks to chat with you via email and tells you he is out of town for his job and you can wire him the monthly rent.
I'm only out $7.95 so it's not the worst thing in the world, but I would not recommend this particular site. Instead, I had success with the Roomi app and Spare Room.